1. Cross-Reference to Related Application
This application is related to a co-pending patent application titled "Differential Comparator With Differential Threshold for Local Area Networks or the Like", by F. J. Fernandez and R. H. Leonowich, Ser. No. 07/810,272, filed simultaneously with, and assigned to the same assignee, as this application.
2. Field of the Invention
Invention relates to line drivers in general and, more particularly, to integrated circuit balanced (differential) line drivers.
3. Description of the Prior Art
Many hard-wired digital communication systems use a twisted pair wires as the communication medium between nodes on a network. For example, CSMA/CD local area network systems (Carrier Sense Multiple Access with Carrier Detect, part of the IEEE 802.3 standard) may use the Attachment Unit Interface standard (using shielded wire pairs) to communicate between users on the network and a coaxial cable backbone. Similarly, 10BASE-T local area networks use unshielded wire pairs for communication between users.
Transmission on a wire pair is usually differential in nature to maximize the signal-to-noise ratio at the far end of the pair. What shape the signals "look like" when transmitted is usually contained by a template which is part of a standard. Using the above example for the Attachment Unit Interface (AUI) standard (part of the IEEE 802.3 standard), a pulse template, pulse diagram, test load, and other voltage limitations/specifications are furnished by a standards body. This information sets the performance limitations on a transmitted pulse. Because the test load is reactive and resistive, significant pulse over/undershoot and ringing can occur. The over/undershoot and ringing are usually considered undersirable and the standard places limits on them.
A widely used prior-art driver for AUI systems is shown in FIG. 3. The driver 30 provides a differential signal to test load 20 (as defined by the standard) through DC-blocking capacitors 18. The driver 30 has a pair of complementary driven conventional CMOS inverters 31, 32 (using exemplary inverter 33) powered between the power supply V.sub.CC and ground. In this instance, however, the power supply return for the drivers 31, 32 is through a variable current source 34 to ground. During data transmission, the inverters 31, 32 are complementary switched to produce the differential signal to load 20. Current source 34 provides more than enough current for inverters 31, 32 to drive the load 20. When the driver 30 is placed in the idle state, current source 34 is throttled back, reducing current flow through load 20. This is required because the AUI standard limits the amount of ringing that can occur when the driver 30 goes from active to idle with the inductive test load 20.
A byproduct of this driver design is a common mode shift that occurs when driver 30 goes idle. The common mode shift (DC) is coupled by capacitors 18 to the load 20 as a pulse. This pulse may exceed that allowed by the standard. Thus, an isolation transformer (not shown), disposed between capacitors 18 and load 20, may be required to meet the common mode shift limitations. Unfortunately transformers have inter-winding capacitance which may defeat the purpose of the transformer, requiring more elaborate compensation techniques (e.g., shunt capacitors to ground which may unacceptably lengthen the data pulse rise/fall times when the driver 30 is active).
Thus, it is desirable to provide a differential driver with less ringing and common mode shift difficulties than prior art drivers. Further, it is desirable to provide such a driver that more closely meets system standards without requiring costly, and potentially unreliable, transformers or other compensation techniques.